It is well known that information can be encoded onto a magnetic medium, such as a magnetic disc or magnetic tape. Such encoding is usually accomplished by generating magnetic flux changes in close proximity to the magnetic medium. The magnetic flux changes can be generated by a magnetic read/write head system. In such a system, electronic signals are converted into magnetic flux by induction at a write gap in a write head. Furthermore, the signals from the magnetic medium are decoded by sensing magnetic flux changes at a read gap in a read head, the magnetic flux changes being produced by the movement of the magnetic medium past the read gap. The magnetic flux changes sensed at the read gap are converted into electronic signals via induction at the read gap.
Conventional magnetic head systems for tape drives comprise at least one read head and one write head. Often, such systems utilize multiple read and/or write heads mounted upon one head block. In an exemplary read/write system, there are three magnetic heads aligned on a head block. This type of head block comprises a forward read head, a write head, and a reverse read head. Normally, the write head is positioned between the two read heads. In the ideal case, the head centers are aligned in a straight line. Ideally, this straight line is also the center line along which the magnetic tape moves, so that there is no error in the alignment of the head block with respect to the center of the tape.
Usually, some error is introduced in the manufacturing process for both the tape cartridge and the head block. This generally unavoidable manufacturing error results in a misalignment of the tape with respect to the edge of the cartridge as well as a misalignment of the individual heads on the head block. In addition to this, the entire head block may be askew by some angle with respect to the base plate of the tape player/recorder.
When reading data in conventional steaming cartridge tape drives, it is necessary to alternate between a forward read head and a reverse read head each time the head block is moved to an adjacent data track. For this reason, it is advantageous to know the displacement error between the forward read head and the reverse read head, so that the read heads may be stepped accordingly as the head block switches to an adjacent data track.
It can be safely assumed for the purposes of the following discussion that the edge of the tape cartridge is aligned with the base plate of the tape player/recorder so that the two form an identical reference line. From this reference line, referred to hereinafter as the B-plane, the angle of displacement of the head block and the angle of displacement of the tape center line can be measured.
Relative to the B-plane, the tape itself may have some characteristic slope which can be specified as a number of minutes of arc, or a number of millimeters per inch. Additionally, the horizontal alignment of the magnetic heads may also have some slope relative to the B-plane. The net effect is that, if one head is centered on a track of the tape, another head which is horizontally displaced from the first head may be "off track" by a significant distance.
Previously, this problem of head-tape misalignment has not been considered a serious deficiency. This is because the tracks on conventional 60-100 Megabyte tapes have been wide enough to allow for such errors. However, with the advent of high density magnetic tapes having storage capacities on the order of 300 Megabytes, track widths have decreased to a point where a small misalignment between the head and the tape can lead to interference with information written on adjacent tracks. For this reason, problems with head-tape misalignment must be addressed.
Prior methods used to correct for head-tape misalignment have been sketchy, approximate techniques. For example, the head block may be moved laterally across the width of the track via a stepper motor until the amplitude of the signal stored on the tape was at a maximum. The distance stepped is then recorded. Thereafter, each time that head was used on that particular track, the head is stepped by the recorded distance. This method is deficient for a number of reasons. First of all, because the information may not be written at the same scaling amplitude over the entire tape, the amplitude may appear to be a maximum at a certain point when it is actually just an inconsistency in the recording. Secondly, since this type of test occurs over a small interval of time, many such tests would have to be performed in order to obtain some average stepping increment. Finally, this method is inexact and it may be difficult to determine where the amplitude of the signal is at its peak.
In another invention, disclosed in European Patent Application No. 87118762.1 (Publication No. 276,451), the edge of the magnetic medium is ascertained. Following this, the write head is moved laterally across the width of the tape some known distance via the stepper motor. The write head then records a data line along the tape. When the data stream is recorded, the edge of the tape is again determined, and the read head is moved up from the edge of the tape, first to the lower edge of the data line, then to the upper edge of the data line. The upper and lower position values of the read head are stored and averaged, so that the read head's theoretical position over the center of the data line is determined. The difference between the read head's averaged center position, and the write head recording position is stored as the error of alignment.
This method also has some inherent disadvantages. First of all, there is the added complexity of recognizing the edge of the tape. Tape edge recognition is susceptible to problems with the consistency of the magnetic medium since, in the manufacturing process, coating the edge of a tape with a magnetic material in a uniform manner is quite difficult. Also, it is often difficult to determine the exact location of the tape edge due to an uncertainty in the width of the track recorded by the write head. Secondly, since the head block must be moved across the tape as measurements are taken, mechanical complexity is introduced in the measuring process. Finally, in the method disclosed in European Patent Application No. 87118762.1 (Publication No. 276,451), the read head must be positioned so that the read head is near the written data line, otherwise the process could use a great deal of tape, as the head moves towards the track one increment at a time.